It is often necessary or desirable for a person to exercise a particular muscle or group of muscles. For example, when a muscle is damaged, such as through injury or surgery, it is important to exercise the muscle to prevent atrophy and to strengthen the muscle for normal use. Further, people exercise healthy muscles to increase strength and to maintain an active and healthy lifestyle, as well as to improve their appearance. Various routines have been developed to exercise different muscles groups by forcing the muscles to contract and extend under a load, such as by moving a free weight against the force of gravity or by moving a pad or a sled whose movement is resisted by an exercise machine.
One exercise that has been developed to exercise the legs and lower torso is known as the squat. The exerciser typically rests a weighted bar on his shoulders. The exerciser then bends his legs (i.e., "squats") to lower the weight (eccentric action) and stands straight to lift the weight (concentric action). This exercise can be dangerous since the exerciser may lose his balance and fall. Further, this exercise can strain the back as the spine will be subject to substantial loads when fully exercising the legs. Another drawback to squats is that they require a partner since they should be done only with a "spotter" to help lift the weight if the exerciser fails. Furthermore, when performing a traditional squat, the weight resistance is constant over the entire exercise stroke. However, the leg muscles have varying strength over the range of motion. Consequently, the muscles are not subject to maximum load over the complete range of motion.
To overcome these disadvantages, machines have been developed to simulate the squat. In one machine, known as a hack squat, a sled is slidingly mounted to a frame at an angle, such as 45.degree. from horizontal. Weight posts, adapted to support free weights, are attached to the sled. The user, once positioned on the sled, exercises by bending and straightening her legs, thereby moving the sled and any weights mounted thereon under the force of gravity. Similar to traditional squats with a barbell, the weight resistance is constant during the exercise stroke. Further, some sort of linear bearings or rollers are required to permit the movement of the sled. Such bearings can have a shorter life than rotational bearings.
In another such machine marketed by the assignee of the instant application, a rail-mounted sled is engaged to a weight stack via a cable and pulley system. The user lies on the sled in a squatting position with his feet on a foot platform. As the user attempts to straighten his legs, the sled moves along the rails, pulling the cable and lifting the weight stack. The user exercises his legs by overcoming the force of gravity on the weight stack. The force resisting the movement of the sled can be changed over the range of motion by implementing an eccentric cam as part of the pulley system. While effective at exercising the legs over the full range of motion, such a machine can be more costly to manufacture and ship because the weight stack is included with the machine.
In another machine, disclosed in U.S. Pat. No. 5,263,914, a leg press machine is provided that has a four-bar linkage to control the movement of a footboard. A linkage mechanism connects the four-bar linkage to a cable and pulley system attached to a weight. As the user presses on the footboard, the four-bar linkage moves the linkage mechanism which, in turn, pulls the cable and lifts the weight. This machine also requires that the weight stack be manufactured and shipped with the machine. Further, the linkage mechanism includes a cable, pulley and lever combination which will require regular, time-consuming maintenance.
In another machine, disclosed in U.S. Pat. No. 5,366,432, a leg press machine is provided that has a four-bar linkage to control the movement of a pushplate. The four-bar linkage is connected to a weight stack by a cable and a series of pulleys. The exercise pushes the pushplate with her feet, rotating the four-bar linkage and lifting the weight. This machine also requires that the weight stack be manufactured and shipped with the machine. Further, the cable and series of pulleys will require regular, time-consuming maintenance.
In another machine, disclosed in U.S. Pat. No. 5,106,080, levers are pivotally mounted to a frame above a seat. Footpads are mounted to each lever. Weight posts, adapted to support removable weight plates, are mounted to each lever. The user exercises by pushing the footpads with his foot, causing the levers to rotate and displacing the weight. The user's foot travels in an arcuate path that is concave up. While this machine is not manufactured or shipped with its own weights, the user's foot exerts force in this unnatural, concave up motion.